Magnetic Random Access Memory (“MRAM”) is a non-volatile memory that is being considered for short-term and long-term data storage. MRAM has lower power consumption than short-term memory such as DRAM, SRAM and Flash memory. MRAM can perform read and write operations much faster (by orders of magnitude) than conventional long-term storage devices such as hard drives. In addition, MRAM is more compact and consumes less power than hard drives. MRAM is also being considered for embedded applications such as extremely fast processors and network appliances.
Consider an MRAM device including a resistive cross point array of memory cells, word lines extending along rows of the memory cells, and bit lines extending along columns of the memory cells. Each memory cell includes a magnetoresistive element (e.g., a spin dependent tunneling junction) having a resistance state of either Rparallel or Ranti-parallel, depending on its magnetization orientation (parallel or anti-parallel). Each magnetoresistive element lies at a cross point of a word line and a bit line. The magnetoresistive elements in this array are coupled together through many parallel paths. The resistance seen at one cross point equals the resistance of the magnetoresistive element at that cross point in parallel with resistances of magnetoresistive elements in the other rows and columns.
Because of this coupling, parasitic or “sneak path” currents can arise during read operations, while resistance states of selected magnetoresistive elements are being sensed. The parasitic currents can interfere with the sensing of resistance states of the magnetoresistive elements, making for unreliable sensing of the resistance state of a single magnetoresistive element in the resistive cross point array.
Blocking elements such as diodes or transistors can be used to block the sneak path block currents. For example, each magnetoresistive element of an MRAM array is connected in series with a blocking element. These blocking elements improve isolation and signal strength, which can increase the reliability of the sensing.
However, the blocking elements tend to be rather large, as they are formed in or on a semiconductor substrate of the MRAM device. The blocking elements can significantly reduce the density of the memory cells, which in turn can increase the size and cost of the MRAM devices.
There is a need to reliably sense the resistance states of the magnetoresistive memory elements in resistive cross point memory cell arrays, without significantly reducing memory cell density.